Clause

Definition and stakes

Henry Watson Fowler,  The King's English (1906)

“ If the long clause beginning with which is intended merely to show how great the weariness is, and which is practically equivalent to so great that, it may be called a defining clause, and the omission of the comma is right. But if the which really acts as a mere connexion to introduce a new fact that the correspondent wishes to record, the clause is non-defining, and the comma ought according to our rule to be inserted before it.
The man, who thinketh in his heart and hath the power straightway (very straightway) to go and do it, is not so common in any country.
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Source: Wikisource

Thomas Wood,  Practical Grammar and Composition

“ A Verb-phrase is a phrase that serves as a verb; as, I am coming, He shall be told, He ought to have been told.
7. A Clause is a group of words containing a subject and a predicate; as, The man that I saw was tall. The clause, that I saw, contains both a subject, I, and a predicate, saw. This clause, since it merely states something of minor importance in the sentence, is called the Subordinate Clause. The Principal Clause, the one making the most important assertion, is, The man was tall. Clauses may be used as adjectives, as adverbs, and as nouns.
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Source: Gutenberg

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